Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications.

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

Sulphate-reducing microbes affect the modern sulphur cycle, and may be quite ancient1,2, though when they evolved is uncertain. These organisms produce sulphide while oxidizing organic matter or hydrogen with sulphate3. At sulphate concentrations greater than 1 mM, the sulphides are isotopically fractionated (depleted in 34S) by 10–40‰ compared to the sulphate, with fractionations decreasing to near 0‰ at lower concentrations2,4,5,6. The isotope record of sedimentary sulphides shows large fractionations relative to seawater sulphate by 2.7 Gyr ago, indicating microbial sulphate reduction7. In older rocks, however, much smaller fractionations are of equivocal origin, possibly biogenic but also possibly volcanogenic2,8,9,10. Here we report microscopic sulphides in ∼3.47-Gyr-old barites from North Pole, Australia, with maximum fractionations of 21.1‰, about a mean of 11.6‰, clearly indicating microbial sulphate reduction. Our results extend the geological record of microbial sulphate reduction back more than 750 million years, and represent direct evidence of an early specific metabolic pathway—allowing time calibration of a deep node on the tree of life.

Isotopic evidence for microbial sulphate reduction in the early Archaean era

P—T—t—deformation—fluid characteristics of lode gold deposits: evidence from alteration systematics

Sedimentary cycling and environmental change in the Late Proterozoic: Evidence from stable and radiogenic isotopes

The δ13C values for Phanerozoic marine carbonates display secular trends, mainly in the range –1 to 2‰1. These have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater, brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1,2. Carbon isotope trends are not as well established for the Precambrian, but there are indications that Upper Proterozoic sequences contain carbonates with unusually high δ13C values3–9. Here we present the results of the first carbon isotopic study of the Upper Proterozoic carbonate strata from the type locality of the Sinian System in the Yangtze Gorges sections near Yichang, South China, and discuss their implications for accumulation of organic matter. We also compare the results for these Chinese strata with δ13C trends in other Upper Proterozoic–Cambrian sequences, and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences.

Palaeoenvironment and carbon isotope stratigraphy of Upper Proterozoic carbonates of the Yangtze Platform

SIGNIFICANT Archaean bedded sulphate deposits have been described only from the Pilbara Block (Western Australia)1,2, the Barberton Mountainland (South Africa)3–6 and southern India7. These are all barite deposits, but there is petrographic evidence suggesting replacement of original evaporitic gypsum in the North Pole (Pilbara) and Barberton deposits. This contrasts with the apparent volcanogenic deposition of barite in the relatively sulphide-rich Big Stubby prospect (Pilbara). This letter summarises the petrographic evidence, concentrating on recent data from North Pole, and reports stable isotopic results for barite and associated sulphides which provide corroborative evidence for the environments of formation of these deposits. Some implications concerning the early hydrosphere, atmosphere and biosphere are outlined.

I. B. Lambert

Papers

Palaeoenvironment and carbon isotope stratigraphy of Upper Proterozoic carbonates of the Yangtze Platform

Stable isotopic compositions of early Archaean sulphate deposits of probable evaporitic and volcanogenic origins

Anomalous sulfur isotope compositions in the Golden Mile, Kalgoorlie